Thermal switch with electromagnetic cycling delay



g- 23, 1955 E. G. FRANKLIN 2,716,176

THERMAL.- SWITCH WITH ELECTROMAGNETIC CYCLING DELAY Filed Sept. 15, 1952 3 Sheets-Sheet l l/VVE/VTOR EDMOND G. FRANKLIN BY W ATTORNEY FIG.2

Aug. 23, 1955 E. G. FRANKLIN 2,716,176

THERMAL SWITCH WITH ELECTROMAGNETIC CYCLING DELAY Filed Sept. 15, 1952 3 Sheetsfiheet 2 Eu -F j I 44 48 I I 54 .108 4 l I! Q1 I26 I20 34 I40 :iililiiiiiihhb w INVENTOR EDMOND G. FRANKLIN y C-W ATTORNEY Aug. 23, 1955 E. c. FRANKLIN 2,716,176

THERMAL SWITCH WITH ELECTROMAGNETIC CYCLING DELAY Filed Sept. 15, 1952 5 Sheets-Shea- 5 )TLLL 208 2K) 212 am 2I6 FIG.9

I90 230 222 I8 224 F 222 gz j I92 FIG. IO F'IG.| I

IIVVENTOR EDMOND G. FRANKLIN BY W ATTORNEY Unitri States THERMAL SWHTQH WITH ELECTROMAGNETIC CYCLlNG DELAY Edmond G. Franklin, Minneapolis, Minn, assignor to General Mills, Inc, a corporation of Delaware This application relates to thermal switches and more particularly to an improved electromagnetic device for delaying the frequency of cycling in such a switch.

Many thermal switches are known in the prior art, which are so sensitive to changes in the temperature of a control member or medium that the switch contacts open and close at a relatively high frequency. This frequent opening and closing causes substantial wear and erosion of the contacts and may result in the maintenance of erratic temperatures of the device. Such frequent opening and closing of the thermal switch is also undesirable in certain types of appliances, particularly where the appliances are subject to limitations relative to the maximum number of cycles per unit of time, as prescribed, for example, by the National Electrical Manufacturers Association. While some electromagnetic devices have been proposed in the past to improve the switch action, such devices have generally used the magnetic attraction as a direct force to hold the contacts together. Thus the magnetic force actually had to be overcome by the thermally responsive portion of the switch before the contacts could separate.

With these problems of the prior art in view it is accordingly one object of the present invention to provide an improved electromagnetic means of delaying the cycling in a thermal switch.

Another object is the provision of an improved electromagnetic device mounted in the thermal switch assembly and designed to increase the contact pressure as the contacts first engage and to increase the contact gap as the contacts first separate, thereby delaying the cycling of the switch and providing cleaner make and break of the contacts, without direct magnetic attraction between the contacts.

A still further object is the provision of an improved thermal switch contact arm assembly in which part of the contact arm includes the core of an electromagnetic device and in which the contact itself is connected to a movable magnetic armature so that the relative position of the contact is changed by energization and de-energization of the coil.

Still another object is the provision of such a thermal switch in which the electromagnetic coil is connected in circuit with the contacts and is energized to move the armature in a direction to increase the contact pressure after the contacts first engage.

Another object is the provision of a complete switch assembly in which the two contact arm units are located in relative positions which contribute to efiicient operation of the magnetic cycling delay device.

Other objects and advantages will be apparent from the following specification in which certain embodiments of the invention are described.

In the drawings which accompany this application, and in which like reference characters indicate like parts, Figure 1 is a side view, with certain portions in section and other portions broken away for clearness, of a thermal switch embodying a preferred form of the present inatent O" 2,716,176 Patented Aug. 23, 1955 vention. In Fig. 1, the switch parts are shown during the cooling portion of the cycle, just before the contacts re-engage each other.

Fig. 2 is a partial perspective view showing details of the contact and armature mounting at one end of the electromagnetic core.

Fig. 3 is a partial perspective view showing details of the mounting of the electromagnetic core on the corresponding switch contact arm, together with details of the limiting stop for the armature.

Fig. 4 is a view similar to Fig. 1 but with the contact arms in the position in which the contacts first engage each other before any substantial current passes therethrough.

Pig. 5 is a view similar to Fig. 4 just an instant after the contacts engage and illustrating the movement of the armature to increase the contact pressure.

Fig. 5 is a view similar to Figs. 4 and 5 showing the position of the parts just as the switch contacts first separate.

Fig. 7 is a view similar to Fig. 6 showing the parts an instant after the contacts have separated, as in Fig. 6, and illustrating the movement of the magnet armature and contact to increase the contact gap on de-energization of the magnet.

Fig. 8 is a view similar to Fig. 1 showing another embodiment of the invention.

Fig. 9 is a partial perspective view showing details of the electromagnet core of the device of Fig. 8.

Fig. 10 is a chart showing typical operating temperatures of a prior art switch similar to the device of Fig. l, but Without the electromagnetic delay device; and

Fig. 11 is a chart showing typical operating temperature cycles of a switch including such delaydevice.

In the preferred embodiment of Figs. 1 through 7, the thermal switch assembly is designated generally at 20. This thermal switch assembly is mounted on a heating device 22 which is illustrated by way of example as a portion of a fiatiron soleplate. A heating element 24, which may be of any suitable form, but is illustrated as a sheet or rod-type heating element cast into the soleplate, serves to heat the soleplate. This heating element 24 is connected in circuit with leads 26 and 28 connected to the thermal switch for control of the heating element in known The switch assembly itself includes a supporting bracket 30 having flanges 32 secured by bolts 34 to appropriate bosses 36 in the soleplate 22. Extending upwardly from the supporting bracket 30 are two rivets or bolts 38 which maintain the switch parts in their assembled relation. Supported by these bolts 38 are upper and lower contact arm assemblies, designated generally as 40 and 42, respectively. The upper contact arm assembly includes a contact 44 carried at the outer end of a resilient con.- tact arm 48. Contact arm 48 is resiliently biased so as to urge contact 44 downwardly at all times. Just above contact arm 48 is a positioning arm 50 having at its outer end a U-shaped section including a down-turned portion 52 and a return-bent end 54 which extends beneath the outer end 56 of contact arm 48 to limit the downward movement of contact 44 under the resilient bias of blade 48. This control or limiting arm 50 is also connected at its inner end to the stack of elements supported by posts 38 and is resiliently biased upwardly. The strength or resilience of member 50 is greater than that of contact arm 48, so that engagement of stop 54 with the end 56 of contact arm 48 will result in upward movement of both the limiting member 50 and contact arm 48 to a predetermined adjusted position.

To determine this initial adjusted position, the switch assembly includes an upper control bracket 58 having depending sides 60 to provide a channel shape of desired rigidity. At the outer end of this control bracket 53 there is a threaded opening 62, which receives the threaded portion 64 of an adjusting or control shaft 66. The lower end of this shaft 66 carries an insulating button 68, which engages the upper surface of the contact positioning arm or member and thus limits the upward movement of member 50 under its resilient bias. In other words, to tation of shaft 66 will cause vertical movement of the insulating stop 68 and thus adjust the vertical position of arms 48 and 50 to establish a predetermined initial position for contact 44.

The various contact arm parts are supported on the posts 38 in resilient relation with respect to each other by means of intermediate insulating members 70, 72, 74, 76 and 78. Thus, insulated member 70 is positioned between the control bracket 58 and the upper contact positioning member 50, while insulator 72 is positioned between the member 50 and the flexible or resilient contact arm 48. The insulator 74 in turn separates the contact arm 48 from the portions of the lower contact arm assembly 42.

This lower contact arm assembly includes a contact arm. having one end 82 supported in the stack of insulators by posts 38. Arm 80 is relatively flexible and is resiliently biased so that its outer end tends to move upwardly toward the upper contact arm assembly.

Connected to the outer end of contact arm 80 is the horizontal end section 83 of an electromagnet core member 84 of paramagnetic material. Portion 83 is upwardly offset from the main core portion 84 and is connected thereto by the vertical section 88. Thus, the core portion 83 and the main core body 84 extend longitudinally in the same general direction as contact arm 80 and are essentially parallel to the upper contact arm assembly previously described. Suitable rivets 86, or other fastening means, secure the core end portion 83 to the outer end of flexible contact arm 80.

The core portion 84 is formed at its outer end with an upwardly extending lip to which is riveted at 92 the lower end 94- of a vertical contact arm bracket portion 96. This bracket includes at its upper end a horizontal flange 98 which carries the lower contact 46.

The vertical portion 96 of the contact supporting bracket is substantially flexible at a point just above the end 90 of the magnetic core. In other words, the bracket is designed to pivot or hinge at this point. This hinging action is facilitated by the provision of a horizontally extending cut-out area 100 (see Fig. 2) which removes a substantial portion of the metal and thus leaves weakened portions 102 at each side of the hinge area. the upper bracket portions 96 and 98 may swing as a rigid unit from the hinge area of these weakened portions 102.

Riveted at 104 to the upper vertical contact portion 96 is the downwardly extending flange 106 of an electromagnet armature 108. Armature 108 extends generally horizontally and parallel to the main magnet core portion 84. The inner end 1141 of this armature 108 extends just above the inner end portion 83 of the magnet core. This inner end is provided with a vertical opening 112 through which a non-magnetic positioning and limiting screw 114 extends. The lower end 116 of this screw 114 is adjustably threaded into the core portion 83, while the upper end of screw 114 has a head 118 which limits the upward movement of armature end 110.

The contact support bracket 96 is so formed that the hinge portion 102 resiliently biases the armature 108 upwardly, so that armature end engages the limiting head 118 of screw 114 whenever the magnet is de-energized and there are no opposing forces involved.

To energize and de-energize the magnetic core, a coil 120 is wound around the main core portion 84. Gne end of the coil is connected at 122 to the end 90 of the coil or to some other portion of the assembly which is in electrical circuit with contact 46. The other end of Thus,

lit

the coil 120 is connected at 124 to a flexible terminal strip 126 mounted in the stack between insulators 76 and 78. The terminal connection 26 for the heating unit 24 is thus connected to this member 126, while the other terminal wire 28 of the heating circuit is connected to an extension 128 on the upper contact arm 48.

To control the position of lower contact arm assembly 42, a control bracket 130 is riveted at 132 to the vertical portion 83 of the magnetic core. This control bracket 130 has a horizontal flange 134 at its lower end and flange 134 in turn carries a pin or rivet 136 by which the bracket is connected to one end 138 of a control member 140. This control member 140 has its oppoosite end 142 secured to a remote portion 144 of the soleplate 22 by means of a bolt 146. Insulators 145 and 147 at this point prevent leakage of current to the soleplate, although it will be understood that such insulation can be located at any other point along the mechanical linkage from bolt 146 to vertical core portion 88.

The coefficient of thermal expansion of member 140 is substantially less than that of the heating device 22, so that the relative differences in thermal expansion between the point of attachment of screw 146 and the point of connection 34 of the switch will cause relative movement of the pin 136 to rock the control bracket 130 and lower contact arm assembly. This operation is well known and will be readily understood. It involves downward movement of the contact arm assembly 42, against the upward bias of contact arm 80, to disengage the contacts 44 and 46 in response to predetermined heating and expansion of member 22. Conversely, when the heating unit 24 is deenergized, member 22 will contract as it cools and thus permit upward rocking movement of contact arm assembly 42 under the resilient biasing action of the contact arm 80.

With the above description of the parts in mind, the operation of the thermal switch assembly can be readily understood by comparison of Figs. 1, 4, 5, 6 and 7. In Fig. 1 the parts are shown during the cooling portion of the cycle while the heating element is de-energized by the opening of contacts 44 and 46. During this portion of the cycle, the relative contraction of member 22 will permit upward rocking of the lower contact arm assembly until the lower contact 46 just engages the upper contact 44 as shown in Fig. 4. During this portion of the cycle, since the contacts have been disengaged, the coil 120, which is in series with the contacts, is de-energized. Magnet armature 108 has thus remained in its upper or first position as illustrated in Figs. 1 and 4, under the resilient biasing action of the hinge portion 102.

Immediately after the position of Fig. 4 is reached, however, the engagement of the contacts will energize coil 120. This energization of the coil and magnet will result in downward attraction of the inner end 110 of the magnet armature so that such armature end rests against the core portion 83. Because the contact bracket portion 98 extends in the opposite direction from the armature end 110, with reference to the hinge axis 102, the downward movement of the armature end will cause upward movement of the contact 46. This upward movement is accommodated as shown in Fig. 5 by a corresponding upward movement of upper contact arm 48. In other words, this arm 43 and its contact 44 are lifted upwardly by the action of the armature 108 so that the end 56 of the contact arm is lifted above the stop 54 on positioning member 50.

This upward movement will thus not only increase the contact pressure and insure firm engagement of the contacts on the make, but will also make it necessary for the lower contact arm assembly 42 to be rocked downwardly a greater distance before the contacts can again disengage. This extra distance is measured by the spacing in Fig. 5 between contact arm end 56 and stop portion 54 of the positioning member 50. This requirement of increased movement before the break will thus delay the disengagement of the contacts and slow the frequency of cycling.

Ultimately, however, the expansion of the soleplate 22 under the action of heating element 24 will rock the lower contact arm assembly into the position illustrated in Fig. 6. Here the parts are illustrated in the position that they occupy just as the contacts begin to disengage. For convenience in understanding the operation, the magnet armature 108 is shown in Fig. 6 in the attracted, or energized, position. It will be understood, however, that almost immediately after the contacts disengage, the magnet will simultaneously de-energize and the magnet armature will move back to its first relative position as shown in Fig. 7. This rocking movement causes corresponding downward movement of the contact 46 to increase the gap between the contacts and thus provide a clean break. This increased contact gap also makes it necessary for the lower contact arm assembly to rock upwardly a greater distance before the contacts again engage in the position of Fig. 4. Thus the cooling portion of the cycle is delayed and the frequencv of cycling still further reduced.

The cycle just described will be repeated during normal operation of the switch, and in each cycle the electromagnetic core and armature arrangement will increase the contact pressure on the make and increase the contact gap on the break. Thus the desired delay in cycling is achieved. At the same time, however, this extra relative movement of the contact will be limited by the maximum permissible movement of the armature (which can be adjusted by screw 114). The contacts can still separate at the break, without the application of extra forces to overcome the magnetic attraction.

Another embodiment of the invention is illustrated in Figs. 8 and 9. Here the switch assembly includes a supporting bracket 148 which carries a supporting post 150 on which the switch elements are assembled. The switch of Fig. 8 includes an upper contact arm assembly carried by a flexible contact arm 160 having its inner end 162 supported on post 150 in insulated relationship. At the top of the stack a control bracket 152, similar to the bracket 68 of Fig. 1, carries an adjusting screw 154 having an insulating portion 156 at its lower end. This insulating portion on the adjusting member 154 thus controls or predetermines the initial position of the upper contact arm assembly 158. In this case, the electromagnetic cycling delay unit is supported on the upper contact arm 160 and is riveted thereto at 164. The magnetic core end 166 which is thus secured projects horizontally as an extension of the contact arm 160. The main body 168 of this core portion has at its outer end a downwardly extending section 170 with a returnbent portion 172. This return bent or U-shaped portion is riveted at 174 to one end 176 of a flexible magnetic armature member 178. The upper contact 180 is carried by this armature 178.

The inner end 182 of armature 178 is biased upwardly by the resilience of the portion 176 of the armature into engagement with a suitable non-magnetic stop 184. The inner end 166 of magnetic core 168 also has a returnbent or U-shaped portion projecting at 186 beneath the inner end 182 of the armature. Thus, when the core portion is energized, by means of coil 188, the armature end 182 will be pulled down against the portion 186 to complete the flux path through the magnet. This downward movement will cause corresponding downward movement of contact 180.

The coil 188 on the main core portion 168 has one end connected at 190 to a portion of the assembly which is in electrical circuit with contact 180. The other end of the coil 188 is connected at 192 to a terminal strip 194 supported by insulators on post 150. The provision of insulators between the terminal strip 194 and base portion 162 of contact arm makes it unnecessary to insulate contact or the coil connection at from the remaining portions of contact arm 160.

The lower contact 196 is carried by a contact arm 198 having its base portion 200 supported by insulators on post 150. Extension 202 of base portion 200 provides a support for the Wires to the heating element. To control the position of the lower contact 196 in response to the temperature conditions of the soleplate or other heated device, bracket 206 is riveted to the lower arm at 204. This bracket includes a downward extension 208 with a horizontal flange 210 which carries a rivet or pin 212 for connection to one end 214 of a control member 216. This control member may be mounted in a manner similar to that illustrated in Fig. 1 for control member 140. Insulating members 218 isolate the contact arm 198 electrically from these control member portions.

Fig. 9 illustrates details of the construction of the magnet coil 188. Here the coil is formed from a rectangular strip of flat electrically conducting stock, the stock preferably being of the type carrying an insulating coating or outer layer. Slots are provided at 222 extending inwardly from one edge of the strip while similarly opposed slots 224 extend from the opposite edge at points midway between the first slots 222. The respective slots extend only partially across the strip, thereby leaving a series of uncut portions at alternate ends of the slots.

These alternate slots thus provide a series of parallel current conducting portions indicated at 226, 228, 230, 232, 234, 236, 238 and 246. Because of the insulating coating on these portions, the alternate strips may be deflected in opposite directions perpendicular to the plane of sheet 220 to provide a passageway through which the magnet core 168 may be inserted. The interlaced arrangement of the magnet core with respect to the adjacent conducting strips effectively prevents shortcircuiting between the edges of adjacent strips and requires the current to follow a substantially helical or serpentine path from one end of the coil to the other. This coil construction is readily manufactured by a stamping operation and is easily assembled with respect to the core.

Operation of the device of Figs. 8 and 9 is substantially similar to that of the preferred embodiment of Figs. 1 through 7, and need not be described in detail. Here again, initial engagement of the contacts energizes the magnet and causes movement of the magnet armature and opposing contact in a direction tending to increase the contact pressure and delay subsequent disengagement of the contacts. Similarly, the initial disengagement of such contacts de-energizes the magnet and permits resiliently biased movement of the armature and contact away from the opposing contact to increase the contact gap and delay the subsequent reengagement of the contacts.

Figs. 10 and 11 illustrate the effectiveness of the device just described. Fig. 10 shows a chart of temperature against time for a switch similar to that of Fig. 1, but without the electromagnetic cycling delay of the present invention. It is noted that the cycles of operatron are very rapid and that the peaks and valleys of the curve are irregular.

In contrast, the curve of Fig. 11 illustrates the substantially slower cycling of a heating device in which the thermal switch corresponds to the construction of Fig. 1 including the cycling delay means. Here, in addition to the slower frequency of cycling, it will be apparent that the peaks of the curve are at substantially the same temperature level, while the valleys of the curve are also more nearly uniform.

The constructions described in the foregoing specification thus accomplished substantially the objects set forth at the beginning of this application and provide an economical and readily constructed electromagnetic mechanism for delaying the cycling frequency in a thermal switch. Since minor variations and changes in the exact details of construction will be apparent to persons skilled in this field, it is intended that this invention shall cover all such changes and modifications as fall Within the spirit and scope of the attached claims.

Now, therefore, I claim:

1. A thermal switch comprising first and second contact arm assemblies having first and second contacts respectively, said first contact arm assembly comprising a movable first contact arm, an electromagnet secured to said arm and having a magnet armature mounted for relative movement between first and second positions, means biasing the armature to its first position when the electromagnet is de-energized, the armature being moved automatically to its second position in response to energization of the electromagnet, said first contact being operatively connected to said armature for limited movement toward the second contact in response to movement of the armature to its second position and for limited return movement of the first contact away from the second contact in response to movement of the armature to its first position, and means energizing the electromagnet while the first and second contacts engage each other and de-energizing the electromagnet while said contacts are disengaged.

2. A thermal switch according to claim 1 having a thermally responsive control member operatively connected to one contact arm assembly and moving the corresponding contact into and out of engagement with the other contact in response to predetermined variations from a desired operating temperature to which said control member is responsive, and a manually adjustable control member operatively connected to the other contact arm assembly, operation of said manually adjustable control member thereby changing the position of said other contact arm assembly and determining the operating temperature of the switch.

3. A thermal switch according to claim 1 in which the magnet includes a core extending outwardly along the movable first contact arm, and the magnet armature is hinged to the outer end of the magnet core and extends inwardly parallel to the core and between the core and the second contact arm assembly, said armature having a contact support bracket carrying said first contact at a point located outwardly beyond the hinged portion of the armature, said armature having its inner end spaced from the inner end of the core toward the second contact arm assembly in the biased first position of the armature and having its inner end magnetically attracted toward the core and away from the second contact arm assembly thereby urging the first contact toward the second contact in response to energization of the electromagnet.

4. A thermal switch according to claim 3 having adjustable means determining the extent of relative movement of the first contact toward and away from the second contact during movement of the armature between first and second positions.

5. A thermal switch according to claim 4 in which said adjustable means limits the range of movement of the inner armature end.

6. A thermal switch according to claim 1 in which the magnet includes a core extending outwardly along the movable first contact arm, and the magnet armature is hinged to the outer end of the magnet core and extends inwardly parallel to the core and between the core and the second contact arm assembly, the core having a return-bent portion at its inner end located between the inner armature end and the second contact arm assembly, said armature carrying said first contact at a point located inwardly of the hinged portion of the armature, said armature having its inner end spaced away from said return-bent core portion in a direction away from the second contact arm assembly in the biased first position of the armature and having its inner end magnetically attracted toward the return-bent core portion and toward the second contact arm assembly thereby urging the first contact toward the second contact in response to energization of the electromagnet.

7. A thermal switch comprising first and second contact arm assemblies, the first contact arm assembly including a first movable contact arm having a first contact thereon, and the second contact arm assembly including a second movable contact arm, a rigid magnet core section extending longitudinally from the outer end of the second arm and having a coil thereon, and a magnet armature mounted on the second arm for relative movement between first and second positions, means biasing the armature to its first position when the coil is deenergized, the armature being magnetically urged to its second position when the coil is energized, a second contact operatively supported by the armature for movement thereby, the second contact being moved relatively toward the first contact on movement of the armature from first to second position, and means energizing the coil when said first and second contacts engage each other and deenergizing the coil when the contacts disengage each other, thereby causing relative movement of the second contact toward the first and increasing the contact pressure in response to initial engagement of the contacts and moving the second contact away from the first and increasing the contact gap in response to initial separation of the contacts.

8. A thermal switch according to claim 7 having a thermally responsive control member operatively connected to one contact arm assembly and moving the corresponding contact into and out of engagement with the other contact in response to predetermined variations from a desired operating temperature, and a manually adjustable control member engaging the other contact arm assembly to establish the position of said assembly and thereby adjust said operating temperature.

9. A thermal switch according to claim 8 in Which the first contact arm assembly also includes a movable positioning arm having a limiting stop engaging the first contact arm and limiting the movement of said contact arm toward the second contact, said first contact arm being resiliently biased toward said second contact and against said stop, and one of said control members engaging the positioning arm and thereby controlling the position of said limiting stop and first contact arm.

10. A thermal switch according to claim 7 in which the magnet armature extends generally parallel to the magnet core section on the side toward the first contact arm assembly, one end of the armature being hinged to one end of the core and the other armature end being movable toward the first contact arm assembly to the biased first position of the armature and being magnetically urged away from the first contact arm assembly to the second position of the armature, and means supporting said second contact beyond said one hinged end of the armature at a point on the opposite side of the hinge area from said movable armature end, thereby moving the second contact toward the first contact arm assembly when the armature is magnetically urged away from the first contact arm assembly.

11. A thermal switch contact arm assembly comprising a flexible arm having an inner base portion for con- 12. A thermal switch contact arm assembly according to claim 11 in which said coil consists of a flat rectangular sheet of electrically conducting material having lateral slots extending inwardly from alternate edges of the sheet to provide a sinuous current path from one end of the sheet to the other, adjacent laterally extending sections of the sheet being deformed in opposite directions out of the plane of the sheet, and said core between said oppositely deformed sections and thereby providing a continuous helical current path around the core from One end of the coil to the other.

References Cited in the file of this patent UNITED STATES PATENTS 

